"Truing" an AR's receiver face?

[constructor]

If the windage adjustment is not centered on a zero wind day, it most likely is a barrel index problem.

A receiver that is out of square .002" side to side and the extension fits loose in the receiver bore will point the barrel off .032" in the length of the barrel. At 100 yards that is 7.2"
Mega and Vltor receivers in my experience have smaller bores. The extensions must be tapped in which holds them straight by the OD of the extension where a loose fit extension is aligned by the flange on the front of the extension and face of the receiver.

[Steel head]

A receiver that is out of square .002" side to side and the extension fits loose in the receiver bore will point the barrel off .032" in the length of the barrel. At 100 yards that is 7.2"
Mega and Vltor receivers in my experience have smaller bores. The extensions must be tapped in which holds them straight by the OD of the extension where a loose fit extension is aligned by the flange on the front of the extension and face of the receiver.

ETA- If the barrel extension is a tight fit to the bore and must be lightly tapped in like is common with Mega receivers the face does not really need to be trued since the receiver bore holds the extension square.

These two statements make me feel better.

The SPR upper I built the barrel had to be tapped into the receiver.
I didn't lap the face.
That upper turned out VERY well and the BUIS and scope are centered in their adjustments and accuracy is fantastic.

[Krampus]

John has also told me that all you need to win a high power comp is a barrel that will shoot 3/4" groups.

Two questions for you since truing the receiver makes no difference.
Why do some guys have to adjust their rear iron sight all the way to one side to get on target?
If the extension is crooked .002" across the face do all of the bolt lugs bare evenly on the lugs of the extension?

1. I am not the author, the author is Bob.
2. I do not agree that lapping is completely unnecessary. I believe it can be preferable when indexing or when the lack of squareness exceeds or limits the adjustability of the sight.
3. Otherwise with regard to accuracy, it sounds like those with the talent to notice any difference do not see it as a high priority.
4. Sight rail, if it is so out of square that it limits adjustability then it is an issue. The incidence of unsatisfactory uppers from tier one shops appears localized to their competitors.
5. Committing additional resources but receiving no additional benefit is impractical.


Krampus

[T2C]

Or a canted or curved forward rail.

That's a fact. I have also seen at least one FSB canted, because the holes were not drilled properly for the tapered pins.

I have not run into a windage issue, because the receiver face was not perfectly square. Not yet.

[Krusty783]

Given the hardness difference between the barrel and the receiver, it would take a significant misalignment of the receiver face to affect the barrel alignment. When the barrel nut is torqued, the barrel extension face would deform any minute high spots.

I would suspect the concentricity/runout of the receiver threads vs the mounting hole for the barrel to be rather important.

[BufordTJustice]

As constructor said, ensuring that the bolt lugs bear equally on the barrel extension is enough, in and of itself, for me to take the 15 minutes required to lap the receiver face (with a cold burr in my hand).

[AR15barrels]

I have mandrels that hold both 223 sized and 308 sized AR receivers in the lathe.

I measure face runout in the lathe like this:


I typically see 0.0015" runout on your average receiver and I have measured a bunch of them.

I make them straight/square in the lathe like this:

[markm]

Do you think the Brownells tool is worth it for us do it yourselfers?

[rubber ducky]

I have one - bought it with the Brownell's lapping compound - and have used it with success. Had one receiver that when zeroed, the rear sight was cranked all the way right - after "truing" the front, it brought the sight just about centered when zeroed.

I've chucked this in my drill press, and found it makes more of a mess than I'd like - it sprays the lapping compound and lube all over - so I've been doing it by hand. It takes a while, but I don't mind. If I had to do a bunch of uppers, I may think otherwise, but for the amount I put together, it doesn't bother me.

And given the numbers that I put together, I pretty much do all of them.

Hope this helps,

rd

[markm]

Hope this helps,

Yes. It does. Thanks.